1. Field of the Invention
The present invention relates to a charge/discharge protection circuit for controlling charging/discharging of a secondary battery.
2. Description of the Related Art
In recent years, a secondary battery has widely been used along with developments of downsized portable electronic devices. One of the most-widely-used secondary batteries includes a lithium secondary battery. However, there have been pointed out many problems regarding the lithium secondary battery in that overcharging or overdischarging causes damage, degradation, and the like in the battery, thereby shortening the service life of the battery.
In order to solve those problems, various charge/discharge protection circuits have been developed. Most of those charge/discharge protection circuits have a circuit configuration in which two MOSFETs connected in series are provided as a charge/discharge control switching device in a charge/discharge path, in which a control circuit using a battery voltage as a power source controls the gate voltages of the two MOSFETs to thereby inhibit charging or discharging.
FIG. 5 shows a conventional charge/discharge protection circuit having the above-mentioned configuration (see JP 2872365 B). Specifically, a charge/discharge control circuit 5 turns off a charging control MOSFET 18 through a charging control terminal 23 upon detecting that a battery voltage of the secondary battery 12 is increased to be equal to or larger than a preset voltage, to thereby shut off a charging current. During when the charging current is shut off, a discharging current flows through a parasitic diode of the charging control MOSFET 18.
Similarly, the charge/discharge control circuit 5 turns off a discharging control MOSFET 19 through a discharging control terminal 24 upon detecting that a battery voltage of the secondary battery 12 is decreased to be equal to or smaller than a preset voltage, to thereby shut off a discharging current. During when the discharging current is shut off, a charging current flows through a parasitic diode of the discharging control MOSFET 19.
According to the above-mentioned circuit configuration, it is necessary to externally provide two MOSFETs to the charge/discharge control circuit 5, which increases the size of the charge/discharge protection circuit. Therefore, the circuit configuration described above has a problem in terms of cost and saving space in footprint required in portable equipment.
Further, there has also been another problem in that the device itself is easy to be degraded because the circuit has a mode in which a charging/discharging current flows through the parasitic diode of the MOSFET.
In order to solve those problems, there has been proposed another charge/discharge protection circuit in which only a single charge/discharge control switching device is employed and a charge/discharge current does not flow through a parasitic diode.
FIG. 6 shows a conventional example of a charge/discharge protection circuit in which a double-throw semiconductor switching device is provided to a charge/discharge path. The double-throw semiconductor switching device is a single device having no parasitic diode and has a double-throw characteristic (see JP 2001-251772 A).
Specifically, the double-throw semiconductor switching device 1 has the drain connected to the secondary battery 12, the source connected to a load/charger 4, and the gate connected to the charge/discharge control circuit 5. The charge/discharge control circuit 5 turns on and off the double-throw semiconductor switching device 1 in accordance with the terminal voltage of the secondary battery 12.
The above-mentioned circuit merely includes the double-throw semiconductor switching device 1 as the charge/discharge switching device, thereby realizing a simpler circuit configuration. In addition, the double-throw semiconductor switching device 1 shuts off currents in both directions when turned off, and therefore the circuit does not have a mode in which a current flows through a parasitic diode, which less likely to cause degradation in the device itself.
However, according to the conventional charge/discharge protection circuit using the double-throw semiconductor switching device, the charge/discharge control circuit 5 cannot detect which one of a load and a charger is connected to the charge/discharge protection circuit. Therefore, there has been a problem in that, once the double-throw semiconductor switching device is turned off in order to shut off a charging current, it is impossible to cause a discharging current to flow even when a load is connected by replacing the charger which has been removed from the charge/discharge protection circuit. There has also been a similar problem in that, once the double-throw semiconductor switching device is turned off in order to shut off a discharging current, it is impossible to get charging started even when a charger is connected to the charge/discharge protection circuit.
In addition, according to the above-mentioned conventional example, in a case where an excessive charging current or an excessive discharging current which may lead to a breakdown of the double-throw semiconductor switching device flows through the charge/discharge path, the charge/discharge control circuit does not have a function of detecting the current and shutting off the excessive current. Therefore, the circuit has a problem in terms of reliability.
Further, according to the above-mentioned conventional example, it is assumed that the double-throw semiconductor switching device is provided as a field effect transistor having a normally-on characteristic. According to the normally-on characteristic, the switch remains on even when no voltage is applied to the gate thereof. Due to the characteristic, in a case where the connection between the control circuit and the gate of the semiconductor switching device is cut off due to an unexpected event, charging or discharging is allowed, resulting in continuation of charging or discharging which may lead to destruction or degradation of the secondary battery. Therefore, the circuit has a problem in terms of protection of the secondary battery.